This application claims the priority benefit of Taiwan application serial no. 89102173, filed Feb. 10, 2000.
1. Field of Invention
The present invention relates to a photolithography technology. More particularly, the present invention related to a multilayer photoresist process in photolithography for increasing photolithography process window magnitude.
2. Description of Related Art
In the semiconductor photolithography, the exposure resolution and the depth of focus are two key parameters that determine photolithography quality. According to Rayleigh""s theory, the exposure resolution and the depth of focus in a photolithography process can be expressed as follows:
Resolution=K1xc3x97xcex/NA
DOF=K2xc3x97xcex/NA2
Where xcex is the wavelength of incident light, NA is the numerical aperture of the lens, and K1 and K2 are factors controlled by both the optical system and the photoresist, These parameter values depend on the photoresist material, the photoresist thickness, and the exposure method.
Generally, the thinner the photoresist layer is, the lower K1 value, the higher K2 value, and the higher exposure margin are. The K1 value is reduced corresponding to the exposure resolution is increased. The K2 value is increased corresponding to the depth of focus is increased. Therefore, the thinner the photoresist layer is, the better the pattern quality is, that is, the larger the lithography process window is. However, the over thin photoresist layer can not usually resist the plasma etching, therefore, the photoresist layer must have a certain thickness to serve as an etching mask.
To resolve the above problems, the bilayer process is used in photolithography process in the prior art. The bilayer process, which is the top layer and the bottom layer with different material are coated on a wafer in sequence. The top photoresist layer is patterned, and subsequently the bottom layer is dry-etched. The top patterned photoresist layer is in combination with the bottom layer to form a thick composite photoresist layer. However, the bilayer method includes overlapping two layers and subsequent patterning the top photoresist layer and dry etching the bottom layer. Therefore, the top photoresist material should be different from the bottom photoresist material in etching selectivity.
Based on the foregoing, the present invention provides a multilayer photoresist process in photolithography to solve the above problems of concurrently taking the photoresist thickness and the pattern quality into consideration. Many thin photoresist layers are accumulated to form a composite photoresist with a desired thickness.
The present invention further provides a bilayer photoresist process in photolithography to solve the problems of the bilayer photoresist process of the prior art.
According to a preferred embodiment of the present invention, the present invention provides a process, comprising the following steps. A photoresist layer is formed on a substrate, and subsequently exposed through a photomask, followed by the developing process to pattern the photoresist. Then, the first patterned photoresist layer is stabilized to avoid the transformation of constituents due to the influence with the subsequent exposing and developing steps of the overlying photoresist layers. This sequence is repeated untill at least another one layer is deposited and patterned on the substrate. Each photoresist layer has almost the same pattern with the underlying patterned photoresist layer. The photoresist layers are accumulated to form a composite photoresist layer with a desired thickness. Photoresists are of two types: positive, which on exposure to light become more soluble in a developer solution, and negative, which become less soluble, and any one photoresist layer in the composite photoresist layer can be positive or negative.
According to the present invention, each photoresist layer has almost the same pattern as the underlying patterned photoresist layer. If both the photoresist layer and the underlying photoresist layer are the same type photoresists such as positive or negative. The same photomask containing the desired pattern is applied to the same type photoresist. If the type of the top photoesist layer is different from the type of the underlying photoresist layer, such that one is positive (negative), and the other is negative (positive). While the two photoresist layers are exposed through the photomasks with patterns, the pattern in one photomask is complementary to that in another photomask.
Accordingly, the present invention provides a method to form a composite photoresist with a desired thickness. The composite photoresist layer is composed of many thin photoresist layers. Every thin photoresist layer is thinner than the composite photoresist layer. The thinner the photoresist layer is, the larger the lithography process window is. Therefore, the large lithography process window is obtained by accumulating many thin layers to a desired thickness rather than by forming a single photoresist layer with the same desired thickness.
In addition, the magnitude of the lithography process window depends on the monolayer thickness. In the present invention, a composite photoresist is composed of many thin photoresist layers to increase the total photoresist thickness. Meanwhile, the magnitude of the lithography process window is kept large.
Furthermore, in the present invention, the photoresist layer is stabilized before the subsequent light-exposure, and photoresist development steps for the overlying photoresist layer. In the case of bilayer photoresist, both the two photoresist layers can use the same material. Therefore, comparing with bilayer process of the prior art, the photoresist material is easily found in the present invention.
Accordingly, many thin photoresist layers are combined in a composite photoresist with a desired thickness to increase the magnitude of the lithography process window. In addition, the magnitude of the lithography process window depends on the monolayer thickness. Therefore, the present invention provides a method to increase the total photoresist thickness, and keep the lithography process window large. Furthermore, In the case of bilayer photoresist process, the photoresist layer is stabilized before subsequent light-exposure, and photoresist development steps for the overlying photoresist layer. Therefore, both the two photoresist layers can use the same material, and therefore the photoresist material and light-exposure conditions are easily found.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.